In recent years, in order to achieve high breakdown voltage, low loss, and the like in a semiconductor device, silicon carbide has begun to be adopted as a material for the semiconductor device. Silicon carbide has a wide energy bandgap, high melting point, low dielectric constant, high breakdown-field strength, high thermal conductivity, and high saturation electron drift velocity compared to silicon. These characteristics would allow silicon carbide power devices to operate at higher temperatures, higher power levels, and with lower specific on-resistance than conventional silicon based power devices. Such devices must also exhibit low reverse leakage currents. Large reverse leakage currents cause premature soft breakdown.
For power applications, silicon carbide's wide bandgap results in a high impact ionization energy. In turn, this allows SiC to experience relatively high electric fields without avalanche multiplication of ionized carriers. By way of comparison, silicon carbide's electric field capacity is about ten times as great as that of silicon.
It has been known that in Schottky diodes the metal semiconductor interface between the Schottky barrier metal and the semiconductor plays a crucial role in the electrical performance of electronic devices. Many factors can worsen the performance of the interface in a Schottky diode. Also, an anneal for diffusion of an implantation of a P-type dopant would require the temperature on the order of 1500 to 2000° C., which raises acute technological problems and the costs of the equipment can be very high.
The elementary structure of a Schottky diode is illustrated in FIG. 1. This diode is formed from a heavily-doped N-type substrate 1 that is formed on an N-type epitaxial layer 2 properly doped to have the desired Schottky threshold. On this epitaxial layer N is deposited silicon oxide/or other dielectrics 3 defining a window in which the Schottky contact is desired to be established by means of an adequate metallization 4. The rear surface of the component is coated with a metallization 5.
Such a structure has a limited breakdown voltage. Indeed, the equipotential surfaces tend to curve up to rise to the surface at the periphery of the contact area, which results, especially in the equipotential surface curving areas, in very high field values, which limit the possible reverse breakdown voltage. To enhance device performance, design features called JBS (Junction Biased Schottky) bars and FGR (Floating Guard Rings) were developed, formed by implantation-diffusion at the periphery of the active area of the Schottky diode. However, as discussed above, the formation of a P-type area is not easy by implanting/diffusing in a structure as a silicon carbide substrate because of the requirement of high energy, elevated temperature implantation, and subsequent high temperature annealing for activation. Any such P type region also introduces junction capacitances that will compromise the device's high frequency response.
Therefore, there remains a need for a new and improved fabrication technique especially on the silicon carbide substrate to overcome the problems stated above.